The United Nations children's agency says attacks by Islamic extremist group Boko Haram in northeastern Nigeria and neighboring countries have forced more than 1 million children out of school heightening the risk they will be abused, abducted or recruited by armed groups.

The agency said Tuesday the conflict has forced 2,000 schools to close, some for more than a year, in Nigeria, Cameroon, Chad and Niger. Some have been looted or set on fire by the militant group whose name means "Western education is sinful."

Insecurity also prevents teachers from going back to classes. And in schools that have reopened, classrooms are overcrowded.

Boko Haram's insurgency has killed about 20,000 people and displaced millions, according to Amnesty International.